The present invention relates to fiber-optic cable and, in particular, to the reinforcement of such cable.
Fiber-optic cable is rapidly gaining in preference over metallic wire electric cables for a variety of reasons, especially the wide band width and low attenuation which are characteristic of optical fibers. Optical fibers, however, are generally very thin and unable to withstand appreciable mechanical loading, and thus do not aid in strengthening the cable as do the metallic wires of electric wire cables. As a result, special measures must be taken to reinforce fiber-optic cable because the transmission capability of optical fibers deteriorates as the fibers are subjected to strain. Below a certain level of strain, the deterioration is reversible and is thus permissible during cable installation, but not during cable operation. Above that level of strain, however, the deterioration is permanent and is thus not permissible at any stage of handling or operation. Thus, it is critical that stresses incurred during handling and operation be prevented from excessively straining the optical fiber.
The provision of strength members in the cable represents one possible means of reinforcing the cable. The use of metal reinforcement has been proposed in U.S. Pat. No. 3,865,466 to Slaughter and U.S. Pat. No. 4,110,001 to Olszewski et al. Other materials, such as nylon, polyester, lyotropic liquid crystalline polymer (e.g., Kevlar poly (p-phenylene terephthalamide)), polyethylene, polyethylene terephthalate, cotton, E and S glass/epoxy rods, etc., have also been proposed, e.g., see U.S. Pat. No. 4,037,922 to Claypoole, U.S. Pat. No. 4,093,342 to Foord et al, and U.S. Pat. No. 4,226,504 to Bellino.
However, the use of metal strengtheners is not compatible with all applications of fiber-optic cable, some of which may specify that the cable be free of electrically conductive components. Conversely, in some applications metallic elements may be tolerable, but may be advantageously eliminated from the point of view of reducing cable weight or increasing the useful temperature range of the cable.
The use of poly (p-phenylene terephthalamide) as the reinforcement has first necessitated the dissolution of the polymer in an appropriate solvent for the same, and the solution spinning of a large number of relatively fine denier fibers (e.g., thousands of filaments) which may optionally be embedded in an appropriate resin (e.g., an epoxy resin) to form the stiffening member. Such poly(p-phenylene terephthalamide) is incapable of melt extrusion and the procedures required to form the reinforcing member are time consuming, and involve considerable expense. Also, the resulting stiffening member because of the fabrication techniques inherently required is only with difficulty amenable to formation into complex cross-sectional configurations.
Reinforcig members available in the prior art which are composed of E and S glass/epoxy rods are commonly formed by pultrusion and have been found to present shortcomings during service within the resulting cable assembly. For instance, such rods may be susceptible to undesirable thermal expansion and contraction and have tended to be unduly inflexible and relatively brittle which may result in cable failure if the cable assembly is sharply bent.
Some reinforcement arrangements while generally serving to prevent excessive deformation of the optical fiber may, under certain conditions, such as temperature change for example, actually contribute to such excessive deformations. That is, the particular linear thermal expansion characteristics of the reinforcement may render the overall coefficient of linear thermal expansion of the cable significantly different from that of the optical fiber. As a result, the optical fiber may be subject to excessive deformation under extreme temperature conditions.
For example, attention is directed to FIGS. 1 and 2 which depict a fiber-optic cable unit 8 wherein an optical fiber 10 is encased within a buffer tube 12 formed of a thermoplastic material. The inner diameter of the tube may be of greater diameter than the outer diameter of the fiber, the space therebetween filled with a water-repelling medium. Additional reinforcement (not shown) would typically be provided e.g., a central high-strength elongated number around which the optical fiber is helically wound, or high-strength wires helically wound around the tube 12), since a conventional thermoplastic tube is too weak to constitute a strength member. This can be expected to result in a condition where the net linear thermal expansion coefficient of the overall cable varies considerably from that of the optical fiber itself. Accordingly, as the temperature increases, the cable tends to expand to a greater extent than the optical fiber, whereby the fiber is strained. One manner of minimizing this problem is to preslacken the fiber, as depicted in FIG. 3, whereby the overall cable can expand to a greater extent than the optical fiber itself, without straining the fiber (i.e., the slack is "taken-up" during cable expansion).
However, the amount of pre-slack which can be "built-into" the cable is limited, due to the fact that during colder temperatures the overall cable contracts to a greater extent than the optical fiber due to the significant difference in the coefficient of linear thermal expansion. Thus, as the temperature decreases, the amount of slack increases due to the lesser extent of contraction of the optical fiber. If this results in the fiber bearing against the wall of the tube (FIG. 4), the "microbending losses" in the fiber are significantly increased, thereby increasing attenuation losses of the fiber.
Therefore, it will be appreciated that a mismatch between the linear thermal expansion coefficient of the optical fiber and the net linear thermal expansion coefficient of the overall cable places limits on the upper and lower temperatures in which the cable may be effectively utilized; the greater the mismatch, the smaller the range of effective utilization.
Another problem occurring in connection with fiber-optic cable relates to the difficulty in repairing a broken cable. When a break occurs, it is presently necessary to locate and identify the damaged optical fiber(s) in order to perform a splicing operation. This procedure is difficult enough due to the small size of the fiber, but is made even more difficult in conventional cables which are cluttered with numerous reinforcing strands. Although it has been previously proposed to position each fiber within its own individual tube, e.g., an extruded polyethylene terephthalate tube, in order to facilitate fiber identification, such an arrangement would add to the size, weight, and internal clutter of the cable.
A further problem relates to the fact that conventional techniques for reinforcing the cable must be adapted to the particular type of cable being produced, i.e., the cable must be redesigned for the particular end uses. One reason for this relatively expensive requirement is that in conventional cables the reinforcement is common to all of the optical fibers. In an effort to deal with this problem, it has been proposed to individually encase each fiber (see U.S. Pat. No. 4,188,088 issued to Andersen et al on Feb. 12, 1980). This is achieved by encasing each fiber within a dumbbell-shaped sheath of flexible polymer material. A separate strengthener strand is embedded within another portion of the sheath. This arrangement, however, does not minimize the bulk and weight problems, nor the thermall-induced strain problems discussed earlier.
It is, therefore, an object of the present invention to minimize or obviate problems of the type discussed above.
A further object of the invention is to minimize the size, weight, and bulk of a fiber-optic cable while maintaining ample strength of the cable.
Another object is to increase the temperature range in which fiber-optic cable may be effectively utilized.
An additional object of the invention is to provide a reinforced fiber-optic cable in which the coefficient of linear expansion of the overall cable closely approximates that of the optical fiber.
A further object of the invention is to facilitate the splicing of fiber-optic cables.
Yet another object of the invention is to provide a cable having multiple optical fibers in which the individual fibers can be easily found and identified.
An additional object of the invention is to enable a given component of fiber-optic cable to perform a multiplicity of functions.
A further object of the invention is to facilitate the manufacture of stength members for use in fiber-optic cable.